1. Field of the Invention
The invention relates to a device for processing and measuring properties of a moving rod of material in the tobacco processing industry comprising a microwave measurement device, which has a microwave resonator through which the rod of material is conveyed or can be conveyed, and a microwave generator having an output frequency f0. The invention further relates to a microwave measurement device for a corresponding device and a method for processing and measuring of properties of a moving rod of material, particularly of the tobacco processing industry, that is conveyed through a microwave resonator, wherein an output frequency f0 is generated.
The invention relates, in particular, to the field of rod formation and rod processing in the tobacco processing industry, that is, the creation of endless cigarette rods and endless filter rods in rod making machines. For example, an endless cigarette rod is created by initially showering tobacco onto a rod conveyor, then enclosing the endless rod of tobacco with a strip of cigarette paper, and then cutting cigarettes of multiple use lengths from the endless rod of tobacco. Forming the endless rod of tobacco or endless filters and the subsequently cutting, or cutting to length, of the rod occurs at high speed. With present day cigarette and filter manufacturing machines, rod speeds are typically 10 m/s, wherein with section lengths of 100 mm, there are 100 cutting cycles per second.
2. Discussion of Background Information
The quality of the cigarettes depends on the state of the tobacco in the endless tobacco rod. For this reason, the moisture and the density of the tobacco in the endless cigarette rod are measured, and the density, in particular, is controlled. Furthermore, the case of sudden or transient signal fluctuations suggests the presence of foreign bodies, wherein the corresponding rod sections are subsequently sorted out.
In modern cigarette manufacturing machines, this occurs using microwave measurement devices that have at least one microwave resonator housing, through which the endless rod of tobacco passes, as is disclosed in the document DE 10 2004 017 597 B4 (and in counterpart U.S. Publication No. 2005/0225332), for example, the disclosures of which are expressly incorporated by reference herein in their entireties. That document discloses a resonator housing with a resonator cavity in the form of a hollow cylinder that is disposed symmetrically to the endless cigarette rod. It provides a coupling in antenna and a coupling out antenna, using which a microwave signal is coupled for inducing into the resonator cavity an oscillation, and a transmitted part is coupled out in turn.
The measurement using a microwave resonator utilizes the physical fact that the resonance curve of the microwave field in the microwave resonator changes with the presence of a rod of material in the microwave resonator. In principle, the complex dielectric constant of the rod of material guided through the resonator is measured. The complex dielectric constant has a real part and an imaginary part, or a magnitude and a phase. The information about the density and the moisture content of the rod are contained in the two parameters of the complex dielectric constant. Changes in the density or moisture content lead to the characteristic change of the two parameters, and with it, to the resonance curve of the microwave resonator.
Compared to the unloaded microwave resonator, the maximum or minimum of a resonance curve shifts to lower frequencies in the presence of a rod of material. In addition, the resonance curve broadens. Changes in the density and changes in the moisture of the rod of material respectively create their own specific changes of the position, height and width of the resonance curve. If at least two measurement variables of the resonance curve are measured, the density and the moisture can be determined independent of each other within the scope of the measurement accuracy and of the correlations of the functional dependencies of the measurement variables from the rod density and rod moisture.
An evaluation circuit for evaluating a microwave resonator-measurement signal is known from the document EP 0 791 823 A2 (and its counterpart U.S. Pat. No. 6,163,158), the disclosures of which are expressly incorporated by reference herein in their entireties. Multiple independent measurement variables are created, in that microwaves with at least two different frequencies, with which a part of the resonance curve is sampled, are supplied to the resonator. Shifts of the resonance are captured by comparison of the resonance curves of the resonator influenced by the material and uninfluenced by material, and the damping is captured by comparison of the amplitudes of the resonator curves at the frequencies of the supplied microwaves. The density and the moisture of the endless rod of tobacco are reconstructed from the amplitude of the measured signals and the gradient of the slope.
The fundamental frequency of the microwave signal is adjusted with respect to the resonance curve for the unloaded microwave resonator so that it is located at the inflection point of one of the slopes of the resonance curve. The at least two modulated frequencies lie above and below the inflection point on the same slope. In a numerical example, 5.79 GHz and 5.81 GHz, that is 5.8 GHz±10 MHz, are named as input frequencies. Switching between the two frequencies occurs every 5 μs, i.e. a frequency of 100 kHz. The microwave output signal is rectified via a circulator and a microwave diode, and further led via an analog to digital converter to an evaluation arrangement.
In practice, this procedure is limited. For measuring on a slope of a resonance curve the measurement is performed at a fixed working frequency, preferably at the point of inflection of the slope of the resonance of the unloaded resonator. In the case of comparatively small quantities of material in the resonator, only small signal changes are observed, whereas with large quantities of material, there are large changes of the signal, which can also lead to oversteering the signal. Small signal changes imply poor accuracy of the measurement and poor discrimination of the rod density and moisture. Therefore, the occurring small signal changes with which the system must still function reliably, represent a high demand for accuracy on the microwave signal processing. Due to this high demand, all components must be produced and assembled with very high accuracy, which results in correspondingly high costs.
Also due to the small signal changes, small changes or drifts of the characteristic values of the microwave circuit components, which can arise due to component aging or temperature fluctuations and other external changes for example, impact the measurement accuracy. Consequently, the exact calibration of the system must be checked frequently, and if necessary, must be repeated.
With the known measurement method, the microwave signals must be rectified. This is performed using microwave diodes, in particular, Schottky diodes. These diodes have individual non-linear and temperature dependent characteristic curves which cause systematic measurement inaccuracies that can be only partially corrected based on temperature measurements. This fact limits the measurement accuracy and requires an individual compensation.
Independent of this, with measurements using a microwave resonator outside of the resonance frequency it must be observed that the shape of the electrical field used for the measurement is not aligned ideally axially outside of the resonance frequency, but rather diagonally in the resonator, and in the shape of the field is also dependent on the fill level, rod density and rod moisture. Consequently, the measurement accuracy is position dependent. For foreign body detection based on the microwave measurement, the reliability of detecting foreign bodies possibly present in the rod of material therefore depends on the position in the rod.